1. Field of the Invention
This invention relates to image processing methods and apparatuses, in which images and objects are drawn and displayed on computer screens by use of blending and rendering techniques.
This application claims priority on Japanese Patent Application No. 2004-20974, the content of which is incorporated herein by reference.
2. Description of the Related Art
Conventionally, various image processing methods using attribute tables are frequently used for displaying images and objects on computer screens in television games and pachinko games (wherein ‘pachinko’ represents a Japanese style of pinball machines incorporating computer screens). Image processing apparatuses using attribute tables are designed in such a way that external CPUs write various attribute data such as sprite attribute data (representing identification numbers, display positions on screens, and magnification/reduction ratios) and line attribute data (representing colors, and coordinates of start points and end points in drawing lines on screens) into attribute tables. As to sprite attribute data read from an attribute table, for example, sprite patterns are read from a pattern memory in accordance with identification numbers and are used for drawing image data thereof in a display buffer in accordance with designated display positions and designated magnification/reduction ratios.
When a plurality of sprites and lines are displayed to be overlapped each other in accordance with attribute data on a computer screen, they are sequentially drawn in a display buffer in accordance with a prescribed priority order that is determined in advance, wherein a sprite to be drawn later is given a higher priority compared with a previously drawn sprite, so that in the area in which these sprites overlap each other, the sprite (or line) to be written later is written to overlap with the previously written sprite.
Japanese Patent Application Publication No. H10-187139 discloses an example of an image processing apparatus (specifically, a sprite display apparatus) in which sprites and lines are sequentially written over each other. Hence, in order to perform so-called alpha (α) blending between background image and ‘grouped’ sprites, which are subjected to grouping, alpha blending must be further performed on sprites, which have been already subjected to alpha blending, in the area in which sprites belonging to the same group overlap each other; therefore, it is very difficult to realize high-quality alpha blending. In short, the conventionally known apparatus is not designed to consider the situation where a plurality of sprites belonging to the same group overlap each other.